1. Field of the Invention
The invention relates to a heat exchanger for liquid/liquid heat exchange of the type having a plurality of parallel vertical tubes which transport a first heat exchange liquid upwardly, a second heat exchange liquid being brought into contact with the outside surface of the tubes.
2. Description of the Prior Art
U.S. Pat. Nos. 4,119,139, 3,991,816 and 4,220,193 disclose such heat exchangers which in addition contain a granular mass (i.e. a particle mass) which is fluidized during operation by the upward flow of the first liquid so as to occupy at least the tubes.
An advantage of these known heat exchangers is that the fluidized granular mass has a cleaning effect on the inner surface of the tubes and in addition provides a considerable improvement in the heat flow between the first liquid and the tube walls. In many cases there is also a need for a high rate of heat transfer between the tube wall and the second heat exchange liquid. In the known apparatus the tubes run within a cylindrical container through which the second liquid is passed, possible in the reverse direction. The total flow cross section for the second liquid within these closed containers and over the outer surface of the tubes is usually greater than the total flow cross section through the tubes, which with comparable volumes of the first and second heat exchange liquids may lead to a significantly lower velocity for the second heat exchange liquid in comparison with the first. The result is that the heat transfer between the tube wall and the second liquid is relatively low.
Although this disadvantage can, at least in theory, be partly overcome by making a large number of corrugations on the outside surfaces of the tubes so that the liquid velocities over the tubes can be raised with a consequent improvement in the heat transfer in the heat exchanger, such an arrangement makes the heat exchanger more complicated and therefore more expensive. In addition more pump power may be necessary to pump the second liquid, while in practice it seems that in fact no significant improvement in heat transfer can be obtained in this way.
Because the heat flow at the inner side of the tube walls is improved by the use of the fluidized granular mass, the velocity of the first liquid can be kept lower if the process should require it. In that case, larger numbers of tubes with larger diameters are necessary in order to transfer the same quantity of liquid. In their turn these larger tubes lead to an extra increased transfer rate of the second heat exchange liquid on the outside of the tubes, with the difficulties already discussed.